Pt 2.2: Shapes in the liquid: the hull of today’s performance dinghy

Note: this is the first part of the section dealing with hull shape. It relies on interviews with designers that were done some time ago, and it could do with some updating and further information. If there’s enough interest in SailCraft being published as a book – and I’ll be putting up a poll to gauge that shortly – sections like this will be updated and revised.

 

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The shape a 12 footer singlehanded skiff designed by Paul Bieker for famous C Class cat sailor Fred Eaton shows many of the characteristics of the modern performance dinghy, as drafted by one of the greatest of modern dinghy designers. Pic from the Bieker site.

The ratios and the numbers are the main factors that drive performance, but when it comes to class racing and and handling, hull shape is all-important. It’s hull shape that largely determines the way a boat moves through waves, the way it survives heavy air, and the last fraction of a percent of boatspeed when the numbers are fixed by rules.

TAKE A BOW

It seems that there are everlasting trends in dinghy design. Construction and gear gets lighter, and that allows bows to get narrower and the point of maximum beam to move aft. Bruce Farr has been known to say that if you lined a fleet up by going from the boat with the finest entry angle to the widest, you’d find they were in the order of fastest boat to slowest. Narrower bows are thought to reduce the bow wave and allow the hull to slice through waves. “In the three prototypes for the Vector skiff, we learned quite a bit” recalls Steve Clark. “The bows got finer and the maximum waterline beam moved aft.  I think this works for several reasons. It makes wave encounters less abrupt, so the boat isn’t slowed by chop both upwind and down. It also had the effect (in my view) of easing the transition to planing.  If one accepts that to start planing a boat has to pass its bow wave, then the shape forward has to be such that you can stick a lot of boat through the wave before you actually have to climb over it. Naturally you make it easy on yourself if you can have a long shallow wave to pass (like an IC or catamaran) but if you are stuck with a relatively heavy boat (and almost any two man boat less than 25′ long will be “heavy”)  it seems that you should keep the waterlines forward very straight and narrow.”

So why is the “sharp end” getting sharper, year by year? Designers of the past probably wanted to create the narrowest possible bow, but they were restricted by old technology. Modern hulls are lighter, especially in the ends, so they need less buoyancy to lift them over waves. Perhaps more important is the huge reduction in rig weight. Because the weight of the rig is so high, it’s got a huge amount of leverage on the bow. Every time the bow hits a wave, the rig forms a massive pendulum, five or ten metres high, that swings the bow around. If you had put a modern fine bow on old boat, the momentum of the massive rig weight swinging overhead would push simply push the nose under many waves.

The influence of modern spinnakers, especially asymmetrics, also had an influence. Although computer models don’t agree, most designers feel that asymetrics provide a powerful lifting force to control downwind nosedives. They also mean that the forward hand can stay well aft during sets, drops and gybes, instead of going forward to the mast to wrestle with the pole. Some designers estimate that this allows them to reduce waterline beam around the mast by about 12mm/ ½”. Even in some more conventional classes, technology has had an impact. The National 12s carry no spinnakers, but they’ve recently gone to self launching jib poles so the forward hand no longer has to go forward.

Steve Clark notes that the International 14’s mid-point measurement may have delayed designers from making bows fine enough. The rule “seems normal enough, to require  minimum waterline beam at the mid point of the hull, but what it in fact has done is drive the bow fairing of I-14s into a shape they would not naturally want to be. The max waterline beam wants to be further aft, and getting I-14 designers to branch away from their successful 14 designs is hard to do.” Since the I-14 has been one of the most influential of dinghies, that rule has had a historic influence on many other boats.

The standard measurement for the sharpness of a bow is the bow or entry “half angle”. It is simply the angle between one side of the bow (at the waterline) and the centerline. Looking at a range of dinghy designs show that the half angle generally becomes narrower as boats get newer and longer. An excellent paper by UK Cherub designer Kevin Ellway, worth reading for many reasons, shows that most modern UK Cherubs have half angles of a bit over 13 degrees. Sixteen Foot Skiffs and 14s like the Schumacher 3 design have half angles of around 12 degrees, while 18 Foot Skiffs used their extra length to reduce bow half angle to around 10 degrees and became upwind demons at the cost of nosediving problems at the top mark.

Some designers, like skiff designer and professional naval architect Rob Widders, measure another entry half angle. It runs between the front end of the chines and the centerline and is normally 2 to 3 degrees wider than the entry half angle, because the chine sits in the more flared section of the topsides.

SECTIONAL SHAPES AND THE PLANING FLAT

In the modern era, the move to wings and narrower hulls has emphasised hull section shapes that pack the greatest volume and dynamic lift into narrower waterlines. In broad terms, most boats have turned further away from the old Vee sections that were a heritage of the Uffa Fox era and outmoded materials. The standard is now U-shaped sections in the bow, sweeping into elliptical sections further aft and then developing a flat area along the keel in the midships and stern areas.

The attraction of the U and elliptical sections is simple geometry. A circular section provides the minimum possible surface area (and therefore, the lowest wetted surface) for a given volume. Elliptical hull sections allow the volume that is necessary to support the boat to be contained in a package with less wetted surface and (in a fairly typical hull) about 2in/5omm less waterline beam.

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The NS14 class was largely responsible for introducing the elliptical sectional shape to Australian development classes. This is a Tequila design from naval architect Stuart Friezer.

If we stick to U-shaped sections as we move aft from the bow, to the area where the boat get wider and flatter, they tend to distort into an ellipse in the form of a U with a flat patch along the keel line. Apart from being the lowest shape for wetted surface, the ellipse creates more dynamic lift than the old Vee shapes. Remember, any surface planing over the water generates lifting force at a right angle. With the old angled Vee sections, much of that lifting force was directed inwards, where it did nothing for performance.

With a flat planing section, all of the lift is directed vertically upwards. That extra efficiency allows a designer to create a boat that can plane earlier, or have narrower sections for the same lift. This combination of ellipse and flat forms the midsection of most modern boats, with the notable exception of the Bethwaite designs. “The flat area off the centreline gives you more dynamic lift, and the elliptical sections have lower drag at the speeds that the NSs, Moths and smaller skiffs sail at” says designer and naval architect Stuart Friezer. Merlin Rocket designer Keith Callaghan points out that the planing patch also reduces keel line rocker (and therefore form drag) for the same displacement.

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The centreline planing flat and narrow waterline shape of the modern-era hull shows clearly on this Friezer NS14 design.

Sometimes the planing patch is subtle, where the flat is merely a slightly straighter section of the graceful curve of the ellipses that create the hull. Other flats are almost brutal. In some it seems almost as if a giant plane has ripped along the boat, tearing off the normal keel line and leaving in its place an area as flat as a table. Where it meets the graceful ellipses of the hull, the junction is hard enough to almost be called a chine. This is not a crude shape, but a product of years of development by professional naval architects. Many such boats are highly successful, a shock to those more comfortable with the idea that flowing lines are fast.

ENTER THE NEW BOW

The modern bow is getting longer and narrower, but at the same time it’s carrying more volume. Designers are achieving this trick by turning away from the old Vee sections, which have little volume. Instead, they are carrying elliptical or U sections right forward to the stem. In part, the shift towards U sections is an inevitable result of bows getting narrower. If the fine-bow boats didn’t have flotation somewhere at the pointy end, they’d just go down like a U Boat. “If you go narrow, you’ve got to have good volume from the middle right to the nose” notes Michael Nash.

The U shape gives the bow the required volume and flotation, but it pushes the bulk down low under the water. Moving the flotation down allows the modern boats to be narrower around the level of the sailing waterline, which is generally agreed to be the critical point for wavemaking. “I think extra width has the most effect at the waterline, so everyone’s making boats as narrow as possible at that point. It doesn’t matter much that they have to put more volume under the water” says Nash. “It’s where the boat is breaking the water that’s important” agrees Thorpe.

“The Bieker boats are fuller low down in the bow, and the extra volume gives you more lift” confirms top 14 sailor and sail designer David Alexander. “They are better upwind in chop; the Bieker will go straight through waves that would stop a Wedge (one of the classic Aussie 14 designs). Downwind, the Bieker is easier to sail because the extra volume and rocker forward allows the bow to lift”.

 

Not all boats follow the high-volume U shape. The best shape and extent of the U and the planing flat under the bow may vary according to the basic speed of the boat. In slow and moderate speed boats powered by hiking crews or a single trapeze, the extra dynamic lift is very valuable and the lower speed of the boat means that the flat area is normally submerged, even in waves. But a faster boat planes faster and higher. If the bow is too flat and U-ed forward, the “slamming” effect as the U or flat rattles over waves may well cost the boat more speed than it will save through the wetted surface reduction and dynamic lift. Not surprisingly, too much flat area along the keel line forward seems to have the same effect as the full, low-chined bow shape on some older Southern Hemisphere designs. The skiff-type Topper Boss had very flat bow sections, which helped it plane very early but slowed it upwind it waves. “As for upwind in a chop, well she doesn’t slice through the water let’s put it like that!” recalls a runner-up in the national titles for the Boss class.

The theme carries through when one looks at the bow shapes of the fastest upwind performers; the 49er and 18. They tend towards very fine, Vee shape bows because they are long boats that travel at such speed that dynamic lift is already plentiful. Their speed increases the impact and rate at which they meet the waves, making it vital to ease the shock. Julian Bethwaite also notes that you have to look at an angle to assess a bow’s wave-handling characteristics. “The top of the wave is normally coming up when it hits the bottom of the boat, so the actual impact is at 30 or 45 degrees. You actually have to look at the boat from that angle, not look at it from the horizontal or the section.”

THE MAST AREA

The area around the mast is generally the deepest part of the boat. This, say many designers, is the area that gives a boat its buoyancy, and a lot of its power. Some designers, especially skiffies, like to put a lot of extra volume in the topsides around the mast area. This far back in the boat, the extra volume doesn’t smack the waves as badly as it would if it was further forward. The extra volume in the topsides will be a long way to leeward, and the buoyancy and planing surface will providing good “leverage” to force the boat upright when the boat is heeled over a long way. Andy Patterson notes that Bethwaite boats (especially the 59er) tend to be very veed and deep under the mast, which “makes them easier to sail, less critical for pitch angle.

ENTER THE SLAB

One of the most widespread shifts in design in the last few years is the emergence of the “slab-sided” hull. Just about every development class that where the topsides shape is not dictated by rules is moving away from flare in the topsides, especially in the bow. The move is largely designed to reduce resistance in waves. “If you only need the volume below the waterline to go sailing on, why have the rest of it?” asks Andy Dovell. “What’s the rest of it doing for you? It’s hitting waves and getting knocked around and knocking you around and slowing you down. So if you come straight off the waterplane and straight up, you can poke through waves much more efficiently than if the hull is flared out”.

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The Stealth 1 International 14, designed by David Lugg, shows the slab sides of the typical modern performance dinghy. Pic from the Western Australian class site.

Designers also feel that slab sides also reduce pitching. When a flared bow is driven into a wave, the extra buoyancy up high lifts the bow abruptly. A slab-sided boat has a more gradual increase in buoyancy as it drives into a wave, so it doesn’t accelerate upwards quickly. “Slab sides minimize hull volume in the topsides, which reduces hull weight and resistance when punching upwind in a chop” agrees Bieker. “They weren’t done in 14’s until I put racks on which made the hull shape independent of the beam necessary for trapezing”.

Top sailors like former International 14 world champion Grant Geddes feel that the switch from flared gunwales to racks also helps to cure nosediving. “Waves go shooting straight through the hole between the gunwhale and the racks, where the water would have hit the underside of the flare and driven the bow down.”.  Racks can also be angled up high in the air, giving the crew more clearance from big waves that could wipe them off a lower conventional gunwale. And as Geddes points out, that has another advantage – when the crew don’t have to heel the boat to lift themselves above the waves, they can sail flatter and therefore faster.

Most designers feel that a bit of topsides flare is still safer, because it provides more buoyancy down to leeward when the boat is heeled. Some of Phil Morrison’s 14s have almost vertical topsides, but he went back to flare with the RS 800, where safety was a key word. “Prototype 1 originally had near vertical topsides, but it made capsize almost instantaneous. Flaring the topsides made the boat only marginally slower, looked nicer (in my opinion) and gave the crews a few moments in which they might be able to recover from an error.” Similarly, the 12 foot skiffs, where massive rigs make stability and the ability to sail fast while heeled into a major issue, still have very wide flare.

MORE TO COME…….

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Author: cthom249

A former sailing journalist and magazine editor, I was lucky enough to grow up in Sydney, one of the world's sailing hotspots and to win national and state championships in classes like J/24s, Windsurfer One Designs, offshore racers, Laser Radial open, Windsurfer OD Masters, Raceboard Masters and Laser Radial Masters, to get into the placings in a few other classes, and do a few Sydney to Hobarts.

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